The invention concerns a process for the determination of hematocrit in whole blood in which the conductivities of whole blood and blood plasma are determined and the hematocrit is determined from these conductivity values.
The invention also concerns an apparatus for the determination of hematocrit with means for the determination of the conductivity of whole blood, means for the determination of the conductivity of plasma and a unit for recording measured values.
Hematocrit is an important physiological parameter which is used in the assessment of the condition of a patient under medical care, particularly in the area of intensive medicine such as open heart surgery and dialysis treatment.
Conductivity measurement belongs to methods for the determination of hematocrit. Means for the determination of hematocrit levels of blood on the basis of conductivity measurement are described, for example, in U.S. Pat. specification No. 4,547,735 as well as the printed publications referred to in this specification and in Biomed. Techn., Volume 27 (1982), Pages 171 to 175. Here, a blood sample is measured in respect of its conductivity between two electrodes, in such a way that sedimentation effects in the blood sample do not falsity the measured value.
A measuring arrangement such as this requires a calibration curve with which the measured conductivity values can be compared.
Moreover, with the known means, the absolute conductivity is measured, not the specific conductivity, so that the constancy of the cell constant is a prerequisite. Incidentally, a temperature stabilization is definitely necessary with this process.
However, the use of such installed apparatus, in which a blood sample taken from a patient is placed between two electrodes becomes problematic if it is necessary to measure blood samples from the patient which have different electrolyte levels. The conductivity of the whole blood is then determined not only from the hematocrit, but also substantially from the conductivity of the medium conducting the current and of the blood liquid or plasma.
This, on the other hand, is strongly correlated to the electrolyte content. That is, it is dependent on the concentrations of the kinds of ions mainly determining the conductivity.
The plasma conductivity varies by about 15% with a variation in sodium ion concentration from 130 to 150 mmol/l with constant potassium of 4 mmol/l. From a calibration curve determined for 130 mmol/l sodium, a hematocrit of 30% would only be indicated as 22% to 23% hematocrit.
In cases of an electrolyte disturbance, calibration curves specific to patients would be necessary according to the hereinbefore described known process. The process fails completely if the electrolyte level changes during the treatment, as for example, in dialysis or the infusion of hyperosmolar or hypoosmolar liquids.
The determination of absolute hematocrit levels in extracorporeal dialysis in which the conductivity of whole blood and of plasma is determined has been described in U.S. Pat. specification No. 4,484,135. In the process the plasma is obtained from whole blood in an ultrafilter and afterwards the conductivity measurement is carried out.
From this it was able to be established that changes of hematocrit are correlated to changes in the blood volume provided that the volume of the erythrocytes remains substantially constant.
This known apparatus, however, has the disadvantage that it is very costly, partly because of the necessity that the plasma has to be obtained first by filtration. In addition, because of the known Gibbs-Donnan Effect, the hemofilter results in a change of the ion concentrations in dependence on the protein content. Such changes in conductivity can lead to small errors of a few percentage points in the determination of hematocrit.
Finally, a process for the determination of hematocrit is disclosed in French Patent Specification No. 2 308 099 in which a quantitatively recorded blood sample is added to a determined amount of electrolyte solution and the electrolyte of the blood sample to be quantitatively determined and the electrolyte of the electrolyte solution are identical. In this process the initial concentrations and the concentration of the electrolyte mixture are determined so that from this the resulting values of the hematocrit can be calculated.